MESSENGER shows how a spacecraft could end neutron lifetime stalemate
(11 June 2020 - Johns Hopkins Applied Physics Laboratory) Neutrons aren’t a model of resilience when it comes to living a single life. Strip one from an atom’s nucleus and it will quickly disintegrate into an electron and a proton.
But scientists can’t determine how quickly, despite decades of trying, and that’s problematic because knowing that lifetime is key to understanding the formation of the elements after the Big Bang.
Now, a team of researchers from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, and Durham University in England has provided a way that could end the decades-long stalemate. Using data from NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, the team shows that the lifetime of a neutron can be measured from space. The findings were reported June 11 in the journal Physical Review Research.
Artist’s schematic of how MESSENGER provided data to estimate neutron lifetime. Cosmic rays striking Venus’ atmosphere eject neutrons that gradually fly into space. As neutrons move to higher altitudes, more time passes, and more neutrons radioactively decay. MESSENGER counted the number of neutrons at various altitudes, allowing scientists to compare neutron numbers across altitudes. Using models, researchers could then estimate the neutron lifetime. (courtesy: Johns Hopkins APL)
“This is the first time anyone has ever measured the neutron lifetime from space,” said Jack Wilson, a scientist at APL and the study’s lead author. “It proves the feasibility of this method, which could one day be the way to resolve this anomaly.”
A persisting mystery
Since the early 1990s, scientists have disagreed about how long lone neutrons last, mainly because the two methods used so far give highly precise results that don’t line up.
The “bottle” method traps neutrons in a bottle and tracks how long they take to radioactively decay, which on average is around 14 minutes and 39 seconds. The “beam” technique instead fires a beam of neutrons and tallies the number of protons created from radioactive decay. On average, this takes about 14 minutes and 48 seconds — nine seconds longer than the bottle method.
Nine seconds isn’t much, but relative to the uncertainty in either method’s measurements — at most two seconds — it’s enormous.
Researchers using the bottle and beam measurements continue working to resolve the discrepancy with their techniques. But since 1990, researchers have discussed an alternative way to measure the neutron lifetime: from space.
Cosmic rays colliding with atoms on a planet’s surface or atmosphere set loose neutrons that gradually wind into outer space against the pull of gravity. The farther the neutrons travel from the planet’s surface, the more time passes, and the more neutrons will radioactively decay. By comparing the number of neutrons at various altitudes, a spacecraft could estimate the neutron lifetime.
No mission or instrument has ever been funded to put the idea into practice. But MESSENGER happened to have the right kind of tool that collected the right kind of data.
“Of all past spacecraft measurements, MESSENGER’s are well suited to measuring the neutron lifetime,” said David Lawrence, an APL planetary scientist and study coauthor.
The spacecraft that could
MESSENGER carried a neutron spectrometer to detect neutrons scattered off hydrogen atoms in water molecules suspected (and later confirmed) to be frozen at Mercury’s poles. On its way to Mercury, though, MESSENGER also collected neutron data for the first time over cloud-strewn Venus.
The spacecraft made observations over a large range of heights above Venus and Mercury. The low-energy neutrons emitted by Venus’ atmosphere move at a few kilometers per second. At MESSENGER’s altitude — a few hundred to a few thousand kilometers above the planet’s surface — the neutrons would have traveled for a time similar to the estimated neutron lifetime.
“It’s like a large bottle experiment, but instead of using walls and magnetic fields, we use Venus’ gravity to confine neutrons for times comparable to their lifetime,” Wilson said.
With funding from the United States Department of Energy Office of Science, the researchers used models to estimate the number of neutrons MESSENGER would detect above Venus for neutron lifetimes between about 10 and 17 minutes.
When the scientists compared the actual number of detected neutrons with the modeled lifetimes, they found 13 minutes provided the best match.
The team estimated that lifetime could be off by about two minutes due to statistical errors and other uncertainties, such as whether the number of neutrons changes during the day or at different latitudes. Yet within these uncertainties, their estimated neutron lifetime agrees with values from the bottle and beam methods.
“This result shows that even using data from a mission designed to do something entirely different, it’s still possible to measure the neutron lifetime from space,” said Jacob Kegerreis, a researcher at Durham University and a coauthor on the study.
The future in space
The new technique clearly is a major departure from the relative ease of laboratory experiments. But because the uncertainties in space-based measurements are unrelated to those in the lab-based methods, the researchers contend the new technique provides a way to break the tie between the existing measurements.
Making measurements that are more precise will require a dedicated space mission, possibly to Venus, since its thick atmosphere and large mass effectively trap neutrons around the planet, the researchers say. The team is working with internal APL support to understand how to accomplish such a mission.
“We ultimately want to design and build a spacecraft instrument that can make a high-precision measurement of the neutron lifetime,” Wilson said, and perhaps finally settle this outstanding mystery.
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